There is a limit to how much gain can be achieved from a single stage amplifier. Single
stage amplifiers also have limits on input and output impedance. Multistage amplifiers
are used to achieve higher gain and to provide better control of input and output
impedances. Two significant advantages that multistage amplifiers have over single
stage amplifiers are flexibility in input and output impedance and much higher gain.
Multistage amplifiers can be divided into two general classes, open-loop and negative
feedback. Open-loop amplifiers are easy to understand and design but are sensitive to
environment and component variations. Negative feedback amplifiers are a bit more
difficult to understand but have the advantage of being much less sensitive to
environment and component variations. This note will focus on the open-loop class. A
good closed-loop amplifier begins with a good open-loop design.
For many amplifier applications it is desirable for the input impedance to be very high.
Thus, it is common for the first amplifier stage to be either a common-collector (a.k.a.
emitter follower) bipolar junction transistor stage or a common-drain (a.k.a. source
follower) or even common-source field effect transistor stage. Sometimes high input
impedance is not important and the first stage may be a common-emitter. Field effect
transistors are normally used only for the input stage and for the specific application of
very high input impedance.
It is also common situation that it is desirable for the output impedance of an amplifier to
be low. A common-collector circuit is typically used. But in some cases there is no need
for very low output impedance and the last stage may be a common-emitter.
For the amplifier stages in-between it is common to employ common-emitter circuits
because those can achieve high voltage gain.
Analysis of multistage amplifiers is performed stage at a time starting with the input stage
and progressing to the output stage. The analysis methods are identical to that of single
stage amplifiers. One point of confusion for students analyzing direct coupled amplifiers
is that the collector resistor for one stage becomes the base resistor for the next stage. In
stages involving common-collector amplifiers some modified approaches, including
some simplifying approximations, are necessary because characteristics of
stages are dependent on external impedances. The student should not be afraid
of approximations since that is routinely done all the time in the profession. An
advantage of closed loop amplifiers is that approximation errors are greatly reduced.
The design of multistage amplifiers begins at the output and progresses backwards to the
input. Initially the number of stages is not known. The design progresses with additional
stages until the requirements are met. It is common for there to be a lot of iteration in the
design and the number of stages might vary with each iteration.
The following table is a summary of some different multistage amplifiers constructions
and their characteristics.
General Characteristics of Typical Multistage Amplifier Structures
Stage Number Characteristics
Rin Rout Voltage gain
Stage Number Characteristics
1 2 3 4 Rin Rout Voltage gain
CE CE Medium Medium High
CE CC Medium Low Medium
CC CE High Medium Medium
CC CC Very high Very low <1
CE CE CE Medium Medium Extremely high
CE CE CC Medium Low Very high
CE CC CE Medium Medium Very high
CE CC CC Medium Very low Medium
CC CE CE High Medium Very high
CC CE CC High Low Medium
CC CC CE Very high Medium Medium
CC CC CC very high Very low <1
CC CE CE CC High Low Very high
Descriptor Rin or Rout Voltage gain
Low less than a few hundred Ohms
Medium A few hundred to a few thousand Ohms less than 50
High a few thousand to a few ten thousand Ohms 50 to 500
Very high many tens of thousands of Ohms 500 to 5000
Extremely high Over one hundred thousand Ohms Over 5,000
AC coupled versus DC coupled stages
The simplest method to construct a multistage amplifier is to cascade several single stage
amplifiers with their usual AC coupling. AC coupling blocks DC paths and makes the
bias design or analysis of each stage simple. A typical example is shown in Figure 1.
Figure 1: Multistage amplifier with AC coupling
The use of AC coupling requires a lot of capacitors and resistors that could be eliminated
with innovative design. The key to this is to arrange for the quiescent voltage at the
output of one stage to be the same as the desired quiescent voltage at the input of the next
stage. Then the AC coupling capacitor and associated bias resistors are not needed. The
bias resistors and thus reduce the gain of the amplifier. An amplifier designed without
these can achieve higher gain and with much fewer parts. The following circuit shows
the first example with unneeded parts removed. Note the simplicity.
Figure 2: Multistage amplifier with DC coupling
Direct coupled amplifiers are a challenge for the designer as the bias analysis and design
calculations are more complicated. It is important to design the amplifier such that the
DC gain is low. But, that is what engineers are paid to do. Using as few parts as needed
to accomplish a desired function lowers the costs for the manufacturer.
A good question to ask and explore is, “Is there an upper bounds to the amount of
amplifier can have?” The answer is yes but there is not a specific value. It
depends on a
variety of factors. One limiting phenomena is random noise which exists in all
electronics. These small voltages often in the nanovolt to microvolt range will dominate
or even saturate the output of the amplifier if the gain is high enough. Depending on the
desired bandwidth and how much noise can be tolerated in the output the practical limit
of gain may range from less than a thousand to many millions. Typical amplifiers in the
audio frequency range that operate on microphone or phonograph pickups have voltage
gains in the one thousand range as that is what is needed. The total voltage gain from
microphone to a several hundred watt speaker system in an auditorium can be in the
50,000 range. The power gain might be in the 120 dB range.
Amplifiers can be either open-loop (no feedback from output to input) or closed-loop
(some of the amplifier output is fed back to the input). In a basic electronics course there
is barely enough time to even discuss open-loop amplifiers. Virtually one hundred
percent of real-world amplifiers are closed loop utilizing negative feedback to reduce
undesirable characteristics of the amplifier. Closed loop amplifiers can achieve a very
specific and stable gain with varying temperature and transistor characteristics as well as
much lower distortion. Many of the challenging bias problems for multistage amplifiers
are eliminated with negative feedback. The mathematics is more complicated (again, that
is what engineers are paid for) and one must first understand open-loop amplifiers before
delving into closed-loop amplifiers.
NPN and PNP transistors are often used in multistage amplifiers for improved
characteristics over what could be achieve by using only one type. Temperature
sensitivity can be greatly reduced using both types in certain circuits such that the
voltage drops practically cancel –thus greatly reducing the effect of temperature.
Each individual voltage drop is very temperature sensitive but the net result is the
subtraction of the two. Use of an NPN common-emitter stage followed by a PNP
common-collector stage (or vice-versa) for the output enables near optimum bias
conditions for both.
The following are some examples of multistage open-loop amplifiers.
Figure 3: High voltage gain amplifier
The circuit in Figure 3 is capable of very high gain. The gain can be up to several ten
thousand if RE1B and RE2B are zero. These resistors are often non-zero to reduce the
gain to a desired level.
Figure 4: High input impedance amplifier
The circuit in Figure 4 features an emitter follower input stage for high input impedance
followed by a common-emitter amplifier for high voltage gain. This feature provides a
much higher power gain than can be achieved with a common-emitter amplifier alone.
This circuit features very low temperature sensitivity because the base-emitter voltage
drops of the two transistors practically cancel.
Figure 5: High input impedance, low output impedance, high voltage gain amplifier
The circuit in Figure 5 is about the ultimate in what is practical to do with direct coupled
amplifiers without negative feedback. This circuit features an emitter follower for the
input stage thus providing high input impedance and an emitter follower for the output
stage thus providing low output impedance. The two common-emitter stages in-between
are capable of very high voltage gain as discussed in the circuit for Figure 3.
The following are some examples of multistage closed-loop amplifiers.
Figure 6: Simple inverting amplifier with feedback
The circuit in Figure 6 features simplicity and very high output linear signal swing thanks
to the negative feedback. The output DC voltage is generally set to VCC/2 by the ratio of
the feedback resistor to the base resistor to ground. The inverting gain is set by the ratio
of the feedback resistor to the input resistor.
Figure 7: High gain inverting amplifier with feedback
The circuit in Figure 7 is a very high gain version of the circuit in Figure 6. Operation is
similar except that much higher gains can be achieved. The open loop gain of the
amplifier (not practical to operate in this mode) is in the many hundreds of thousands.
Figure 8: Non-inverting amplifier with feedback
The circuit in Figure 8 is an example of in-phase feedback to boost input impedance
while lowering output impedance. The feedback stabilizes the DC bias and voltage gain.
Issues in Design of Multi-stage Amplifiers:
Why do we need multistages?
• Compared to single stage amplifier, multistage amplifiers provide
increased input resistance, reduced output resistance, increased gain, and
increased power handling capability
• Multistage amplifiers commonly implemented on integrated circuits
where large numbers of transistors with common (matched) parameters
• Typical inverter (Common Emitter) has moderately large gain and has
input and output resistances in the Kilohm range.
• Follower configuration has much higher input resistance, lower output
resistance but has only unity gain.
• Amplifier requires the desirable features of both configurations
Desirable conponent single-stage characteristics
Differential to Single-ended Conversion
DC Level Shifting:
In DC coupled multistage cascade the output bias level of each stage
increases to maintain the collector more positive than the base (constant
current operation). If this voltage ìstackingî is severe, little swing room is
left in the final stages of the cascade
Use of Zener or (ordinary) diode level-shifters (source):
Three stage amplifier with one diode
Three stage amplifier with two diodes
• Use of complimentary npn/pnp BJTs:
A two stage Tranconductance Amplifier
• Use of complimentary NMOS/PMOS FETs:
A two stage CS amplifier (source)
The term amplifier as used in this article can mean either a circuit (or stage) using a
single active device or a complete system such as a packaged audio hi-fi amplifier.
A practical amplifier circuit
An electronic amplifier is a device for increasing the power of a signal.
It does this by taking energy from a power supply and controlling the output to match the
input signal shape but with a larger amplitude. In this sense, an amplifier may be
considered as modulating the output of the power supply.
Types of amplifier
Amplifiers can be specified according to their input and output properties. They have
some kind of gain, or multiplication factor relating the magnitude of the output signal to
the input signal. The gain may be specified as the ratio of output voltage to input voltage
(voltage gain), output power to input power (power gain), or some combination of
current, voltage and power. In many cases, with input and output in the same units, gain
will be unitless (although often expressed in decibels); for others this is not necessarily
so. For example, a transconductance amplifier has a gain with units of conductance
(output current per input voltage). The power gain of an amplifier depends on the source
and load impedances used as well as its voltage gain; while an RF amplifier may have its
impedances optimized for power transfer, audio and instrumentation amplifiers are
normally employed with amplifier input and output impedances optimized for least
loading and highest quality. So an amplifier that is said to have a gain of 20 dB might
have a voltage gain of ten times and an available power gain of much more than 20 dB
(100 times power ratio), yet be delivering a much lower power gain if, for example, the
input is a 600 ohm microphone and the output is a 47 kilohm power amplifier's input
In most cases an amplifier should be linear; that is, the gain should be constant for any
combination of input and output signal. If the gain is not constant, e.g., by clipping the
output signal at the limits of its capabilities, the output signal will be distorted. There are
however cases where variable gain is useful.
There are many types of electronic amplifiers, commonly used in radio and television
transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and
other electronic digital equipment, and guitar and other instrument amplifiers. Critical
components include active devices, such as vacuum tubes or transistors. A brief
introduction to the many types of electronic amplifier follows.
The term "power amplifier" is a relative term with respect to the amount of power
delivered to the load and/or sourced by the supply circuit. In general a power amplifier is
designated as the last amplifier in a transmission chain (the output stage) and is the
amplifier stage that typically requires most attention to power efficiency. Efficiency
considerations lead to various classes of power amplifier based on the biasing of the
output transistors or tubes: see power amplifier classes.
Power amplifiers by application
• Audio amplifier#PowerAudio power amplifiers
• RF power amplifier, such as for transmitter final stages (see also: Linear
• Servo motor controllers, where linearity is not important.
Power amplifier circuits
Can be divided into:
• Vacuum tube/Valve, Hybrid or Transistor power amplifiers
• Push-pull output or Single-ended output stages
According to Symons, while semiconductor amplifiers have largely displaced valve
amplifiers for low power applications, valve amplifiers are much more cost effective in
high power applications such as "radar, countermeasures equipment, or communications
equipment" (p. 56). Many microwave amplifiers are specially designed valves, such as
the klystron, gyrotron, traveling wave tube, and crossed-field amplifier, and these
microwave valves provide much greater single-device power output at microwave
frequencies than solid-state devices
Valves/tube amplifiers also have niche uses in other areas, such as
• Electric guitar amplification
• in Russian military aircraft, for their EMP tolerance
• niche audio for their sound qualities (recording, and audiophile equipment)
Main articles: Transistor, Bipolar junction transistor, Audio amplifier, and MOSFET
The essential role of this active element is to magnify an input signal to yield a
significantly larger output signal. The amount of magnification (the "forward gain") is
determined by the external circuit design as well as the active device.
Many common active devices in transistor amplifiers are bipolar junction transistors
(BJTs) and metal oxide semiconductor field-effect transistors (MOSFETs).
Applications are numerous, some common examples are audio amplifiers in a home
stereo or PA system, RF high power generation for semiconductor equipment, to RF and
Microwave applications such as radio transmitters.
Transistor-based amplifier can be realized using various configurations: for example with
a bipolar junction transistor we can realize common base, common collector or common
emitter amplifier; using a MOSFET we can realize common gate, common source or
common drain amplifier. Each configuration has different characteristic (gain,
Operational amplifiers (op-amps)
Main articles: Operational amplifier and Instrumentation amplifier
An operational amplifier is an amplifier circuit with very high open loop gain and
differential inputs which employs external feedback for control of its transfer function or
gain. Although the term is today commonly applied to integrated circuits, the original
operational amplifier design was implemented with valves.
Fully differential amplifiers (FDA)
Main article: Fully differential amplifier
A fully differential amplifier is a solid state integrated circuit amplifier which employs
external feedback for control of its transfer function or gain. It is similar to the
operational amplifier but it also has differential output pins.
These deal with video signals and have varying bandwidths depending on whether the
video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the
bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB
for example) the bandwidth is measured. Certain requirements for step response and
overshoot are necessary in order for acceptable TV images to be presented.
Oscilloscope vertical amplifiers
These are used to deal with video signals to drive an oscilloscope display tube and can
have bandwidths of about 500 MHz. The specifications on step response, rise time,
overshoot and aberrations can make the design of these amplifiers extremely difficult.
One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.
Main article: Distributed Amplifier
These use transmission lines to temporally split the signal and amplify each portion
separately in order to achieve higher bandwidth than can be obtained from a single
amplifying device. The outputs of each stage are combined in the output transmission
line. This type of amplifier was commonly used on oscilloscopes as the final vertical
amplifier. The transmission lines were often housed inside the display tube glass
Switched mode amplifiers
These nonlinear amplifiers have much higher efficiencies than linear amps, and are used
where the power saving justifies the extra complexity.
Negative resistance devices
Negative resistances can be used as amplifiers, such as the tunnel diode amplifier.
Travelling wave tube amplifiers
Main article: Traveling wave tube
Traveling wave tube amplifiers (TWTAs) are used for high power amplification at low
microwave frequencies. They typically can amplify across a broad spectrum of
frequencies; however, they are usually not as tunable as klystrons.
Main article: Klystron
Klystrons are vacuum-devices that do not have as wide a bandwidth as TWTAs. They
generally are also much heavier than TWTAs, and are therefore ill-suited for light-weight
mobile applications. Klystrons are tunable, offering selective output within their specified
Musical instrument (audio) amplifiers
Main articles: Instrument amplifier and Audio amplifier
An audio amplifier is usually used to amplify signals such as music or speech. Several
factors are especially important in the selection of musical instrument amplifiers (such as
guitar amplifiers) and other audio amplifiers (although the whole of the sound system -
components such as microphones to loudspeakers - will impact on these parameters):
• Frequency response - not just the frequency range but the requirement that the
signal level varies so little across the audible frequency range that the human ear
notices no variation. A typical specification for audio amplifiers may be 20 Hz to 20
kHz +/- 0.5dB.
• Power output - the power level obtainable with little distortion, to obtain a
sufficiently loud Sound pressure level from the loudspeakers.
• Low Distortion - all amplifiers and transducers will distort to some extent; they
cannot be perfectly linear, but aim to pass signals without affecting the harmonic
content of the sound more than the human ear will tolerate. That tolerance of
distortion, and indeed the possibility that some "warmth" or second harmonic
distortion (Tube sound) improves the "musicality" of the sound, are subjects of great
Classification of amplifier stages and systems
There are many alternative classifications that address different aspects of amplifier
designs, and they all express some particular perspective relating the design parameters to
the objectives of the circuit. Amplifier design is always a compromise of numerous
factors, such as cost, power consumption, real-world device imperfections, and a
multitude of performance specifications. Below are several different approaches to
Input and output variables
The four types of dependent source; control variable on left, output variable on right
Electronic amplifiers use two variables: current and voltage. Either can be used as input,
and either as output leading to four types of amplifiers. In idealized form they are
represented by each of the four types of dependent source used in linear analysis, as
shown in the figure, namely:
Input Output Dependent source Amplifier type
I I current controlled current source CCCS current amplifier
I V current controlled voltage source CCVS transresistance amplifier
V I voltage controlled current source VCCS transconductance amplifier
V V voltage controlled voltage source VCVS voltage amplifier
Each type of amplifier in its ideal form has an ideal input and output resistance that is the
same as that of the corresponding dependent source:
Amplifier type Dependent source Input impedance Output impedance
Current CCCS 0 ∞
Transresistance CCVS 0 0
Transconductance VCCS ∞ ∞
Voltage VCVS ∞ 0
In practice the ideal impedances are only approximated. For any particular circuit, a
small-signal analysis is often used to find the impedance actually achieved. A small-
signal AC test current Ix is applied to the input or output node, all external sources are set
to AC zero, and the corresponding alternating voltage Vx across the test current source
determines the impedance seen at that node as R = Vx / Ix.
Amplifiers designed to attach to a transmission line at input and/or output, especially RF
amplifiers, do not fit into this classification approach. Rather than dealing with voltage or
current individually, they ideally couple with an input and/or output impedance matched
to the transmission line impedance, that is, match ratios of voltage to current. Many real
RF amplifiers come close to this ideal. Although, for a given appropriate source and load
impedance, RF amplifiers can be characterized as amplifying voltage or current, they
fundamentally are amplifying power.
One set of classifications for amplifiers is based on which device terminal is common to
both the input and the output circuit. In the case of bipolar junction transistors, the three
classes are common emitter, common base, and common collector. For field-effect
transistors, the corresponding configurations are common source, common gate, and
common drain; for triode vacuum devices, common cathode, common grid, and common
plate. The output voltage of a common plate amplifier is the same as the input (this
arrangement is used as the input presents a high impedance and does not load the signal
source, although it does not amplify the voltage), i.e., the output at the cathode follows
the input at the grid; consequently it was commonly called a cathode follower. By
analogy the terms emitter follower and source follower are sometimes used.
Unilateral or bilateral
When an amplifier has an output that exhibits no feedback to its input side, it is called
'unilateral'. The input impedance of a unilateral amplifier is independent of the load, and
the output impedance is independent of the signal source impedance.
If feedback connects part of the output back to the input of the amplifier it is called a
'bilateral' amplifier. The input impedance of a bilateral amplifier is dependent upon the
load, and the output impedance is dependent upon the signal source impedance.
All amplifiers are bilateral to some degree; however they may often be modeled as
unilateral under operating conditions where feedback is small enough to neglect for most
purposes, simplifying analysis (see the common base article for an example).
Negative feedback is often applied deliberately to tailor amplifier behavior. Some
feedback, which may be positive or negative, is unavoidable and often undesirable,
introduced, for example, by parasitic elements such as the inherent capacitance between
input and output of a device such as a transistor and capacitative coupling due to external
wiring. Excessive frequency-dependent positive feedback may cause what is supposed to
be an amplifier to become an oscillator.
Linear unilateral and bilateral amplifiers can be represented as two-port networks.
Inverting or non-inverting
Another way to classify amps is the phase relationship of the input signal to the output
signal. An 'inverting' amplifier produces an output 180 degrees out of phase with the
input signal (that is, a polarity inversion or mirror image of the input as seen on an
oscilloscope). A 'non-inverting' amplifier maintains the phase of the input signal
waveforms. An emitter follower is a type of non-inverting amplifier, indicating that the
signal at the emitter of a transistor is following (that is, matching with unity gain but
perhaps an offset) the input signal.
This description can apply to a single stage of an amplifier, or to a complete amplifier
Other amplifiers may be classified by their function or output characteristics. These
functional descriptions usually apply to complete amplifier systems or sub-systems and
rarely to individual stages.
• A servo amplifier indicates an integrated feedback loop to actively control the
output at some desired level. A DC servo indicates use at frequencies down to DC
levels, where the rapid fluctuations of an audio or RF signal do not occur. These are
often used in mechanical actuators, or devices such as DC motors that must maintain
a constant speed or torque. An AC servo amp can do this for some ac motors.
• A linear amplifier responds to different frequency components independently,
and does not generate harmonic distortion or Intermodulation distortion. A nonlinear
amplifier does generate distortion (e.g. the output is a current to a lamp that must be
either fully on or off, but the input is continuously variable; or the amplifier is used in
an analog computer where a special transfer function, such as logarithmic, is desired;
or a following tuned circuit will remove the harmonics generated by a non-linear RF
amplifier). Even the most linear amplifier will have some nonlinearities, since the
amplifying devices - transistors and vacuum tubes - follow non-linear power laws
such as square-laws and rely on circuitry techniques to reduce their effects.
• A wideband amplifier has a precise amplification factor over a wide range of
frequencies, and is often used to boost signals for relay in communications systems.
A narrowband amp is made to amplify only a specific narrow range of frequencies,
to the exclusion of other frequencies.
• An RF amplifier refers to an amplifier designed for use in the radio frequency
range of the electromagnetic spectrum, and is often used to increase the sensitivity of
a receiver or the output power of a transmitter.
• An audio amplifier is designed for use in reproducing audio frequencies. This
category subdivides into small signal amplification, and power amps which are
optimised for driving speakers, sometimes with multiple amps grouped together as
separate or bridgeable channels to accommodate different audio reproduction
requirements. Frequently used terms within audio amplifiers include:
o preamplifier (preamp), that may include phono or gramophone preamp
with equalization for RIAA LP recordings, or tape head preamps with CCIR
equalisation filters; they may include filters or tone control circuitry.
o power amplifier (normally assumed to drive loudspeakers), headphone
amplifiers, and public address amplifiers.
o stereo amplifiers imply two channels of output (left and right), although
the term simply means "solid" sound (referring to three-dimensional) - so
quadraphonic stereo was used for amplifiers with 4 channels; 5.1 and 7.1 systems
refer to Home theatre systems with 5 or 7 normal spacial channels, plus a
subwoofer channel (that is not very directional).
• Buffer amplifiers, that may include emitter followers, provide a high impedance
input for a device (perhaps another amplifier, or perhaps an energy-hungry load such
as lights) that would otherwise draw too much current from the source. Line drivers
are a type of buffer intended to feed long or interference-prone interconnect cables,
possibly with differential outputs if driving twisted pairs of cables.
• A special type of amplifier is widely used in instruments and for signal
processing, among many other varied uses. These are known as operational
amplifiers or op-amps. This is because this type of amplifier is used in circuits that
perform mathematical algorithmic functions, or "operations" on input signals to
obtain specific types of output signals. A typical modern op-amp has differential
inputs (one "inverting", one "non-inverting") and one output. An idealised op-amp
has the following characteristics:
o Infinite input impedance (so as to not load circuitry it is sampling as a
o Zero output impedance
o Infinite gain
o Zero propagation delay
The performance of an op-amp with these characteristics would be entirely
defined by the (usually passive) components forming a negative feedback loop
around it, that is, the amplifier itself has no effect on the output.
Today, op-amps are usually provided as integrated circuits, rather than
constructed from discrete components. All real-world op-amps fall short of the
idealised specification above – but some modern components have remarkable
performance and come close in some respects.
Interstage coupling method
Amplifiers are sometimes classified by the coupling method of the signal at the
input, output, or between stages. Different types of these include:
Resistive-capacitive (RC) coupled amplifier, using a network of resistors and
By design these amplifiers cannot amplify DC signals as the capacitors block the
DC component of the input signal. RC-coupled amplifiers were used very often in
circuits with vacuum tubes or discrete transistors. In the days of the integrated
circuit a few more transistors on a chip are much cheaper and smaller than a
Inductive-capacitive (LC) coupled amplifier, using a network of inductors
This kind of amplifier is most often used in selective radio-frequency circuits.
Transformer coupled amplifier, using a transformer to match impedances
or to decouple parts of the circuits
Quite often LC-coupled and transformer-coupled amplifiers cannot be
distinguished as a transformer is some kind of inductor.
Direct coupled amplifier, using no impedance and bias matching
This class of amplifier was very uncommon in the vacuum tube days when the
anode (output) voltage was at greater than several hundred volts and the grid
(input) voltage at a few volts minus. So they were only used if the gain was
specified down to DC (e.g., in an oscilloscope). In the context of modern
electronics developers are encouraged to use directly coupled amplifiers
Depending on the frequency range and other properties amplifiers are designed according
to different principles.
# Frequency ranges down to DC are only used when this property is needed. DC
amplification leads to specific complications that are avoided if possible; DC-blocking
capacitors are added to remove DC and sub-sonic frequencies from audio amplifiers.
# Depending on the frequency range specified different design principles must be used.
Up to the MHz range only "discrete" properties need be considered; e.g., a terminal has
an input impedance.
# As soon as any connection within the circuit gets longer than perhaps 1% of the
wavelength of the highest specified frequency (e.g., at 100 MHz the wavelength is 3 m,
so the critical connection length is approx. 3 cm) design properties radically change. For
example, a specified length and width of a PCB trace can be used as a selective or
# Above a few 100 MHz, it gets difficult to use discrete elements, especially inductors. In
most cases PCB traces of very closely defined shapes are used instead.
The frequency range handled by an amplifier might be specified in terms of bandwidth
(normally implying a response that is 3 dB down when the frequency reaches the
specified bandwidth), or by specifying a frequency response that is within a certain
number of deciBels between a lower and an upper frequency (e.g. "20 Hz to 20 kHz plus
or minus 1 dB").
Type of load
• Tuned (RF amps) - used for amplifying a single radio
frequency or a band of frequencies
Amplifiers are implemented using active elements of different kinds:
# The first active elements were relays. They were for example used in transcontinental
telegraph lines: a weak current was used to switch the voltage of a battery to the outgoing
# For transmitting audio, carbon microphones were used as the active element. This was
used to modulate a radio-frequency source in one of the first AM audio transmissions, by
Reginald Fessenden on Dec. 24, 1906.
# In the 1960s, the transistor started to take over. These days, discrete transistors are still
used in high-power amplifiers and in specialist audio devices.
# Up to the early 1970s, most amplifiers used vacuum tubes. Today, tubes are used for
specialist audio applications such as guitar amplifiers and audiophile amplifiers. Many
broadcast transmitters still use vacuum tubes.
# Beginning in the 1970s, more and more transistors were connected on a single chip
therefore creating the integrated circuit. Nearly all amplifiers commercially available
today are based on integrated circuits.
For exotic purposes, other active elements have been used. For example, in the early days
of the communication satellite parametric amplifiers were used. The core circuit was a
diode whose capacity was changed by an RF signal created locally. Under certain
conditions, this RF signal provided energy that was modulated by the extremely weak
satellite signal received at the earth station. The operating principle of a parametric
amplifier is somewhat similar to the principle by which children keep their swings in
motion: as long as the swing moves you only need to change a parameter of the swinging
entity; e.g., you must move your center of gravity up and down. In our case, the capacity
of the diode is changed periodically.
Power amplifier classes
Angle of flow or conduction angle
Power amplifier circuits (output stages) are classified as A, B, AB and C for analog
designs, and class D and E for switching designs based upon the conduction angle or
angle of flow, Θ, of the input signal through the (or each) output amplifying device, that
is, the portion of the input signal cycle during which the amplifying device conducts. The
image of the conduction angle is derived from amplifying a sinusoidal signal. (If the
device is always on, Θ = 360°.) The angle of flow is closely related to the amplifier
power efficiency. The various classes are introduced below, followed by more detailed
discussion under individual headings later on.
100% of the input signal is used (conduction angle Θ = 360° or 2π); i.e., the active
element remains conducting (works in its "linear" range) all of the time. Where
efficiency is not a consideration, most small signal linear amplifiers are designed
as class A. Class A amplifiers are typically more linear and less complex than
other types, but are very inefficient. This type of amplifier is most commonly
used in small-signal stages or for low-power applications (such as driving
headphones). Subclass A2 is sometimes used to refer to vacuum tube class A
stages where the grid is allowed to be driven slightly positive on signal peaks,
resulting in slightly more power than normal class A (A1; where the grid is
always negative), but incurring more distortion.
50% of the input signal is used (Θ = 180° or π; i.e., the active element works in its
linear range half of the time and is more or less turned off for the other half). In
most class B, there are two output devices (or sets of output devices), each of
which conducts alternately (push–pull) for exactly 180° (or half cycle) of the
input signal; selective RF amplifiers can also be implemented using a single
These amplifiers are subject to crossover distortion if the transition from one
active element to the other is not perfect, as when two complementary transistors
(i.e., one PNP, one NPN) are connected as two emitter followers with their base
and emitter terminals in common, requiring the base voltage to slew across the
region where both devices are turned off.
Here the two active elements conduct more than half of the time as a means to
reduce the cross-over distortions of class B amplifiers. In the example of the
complementary emitter followers a bias network allows for more or less quiescent
current thus providing an operating point somewhere between class A and class
B. Sometimes a figure is added (e.g., AB1 or AB2) for vacuum tube stages where
the grid voltage is always negative with respect to the cathode (class AB1) or may
be slightly positive (hence drawing grid current, adding more distortion, but
giving slightly higher output power) on signal peaks (class AB2). Solid state class
AB amplifier circuits are one of the most popular amplifier topologies used today.
Less than 50% of the input signal is used (conduction angle Θ < 180°). The
advantage is potentially high efficiency, but a disadvantage is high distortion.
Main article: Switching amplifier
These use switching to achieve a very high power efficiency (more than 90% in
modern designs). By allowing each output device to be either fully on or off,
losses are minimized. The analog output is created by pulse-width modulation;
i.e., the active element is switched on for shorter or longer intervals instead of
modifying its resistance. There are more complicated switching schemes like
sigma-delta modulation, to improve some performance aspects like lower
distortions or better efficiency.
There are several other amplifier classes, although they are mainly variations of
the previous classes. For example, class G and class H amplifiers are marked by
variation of the supply rails (in discrete steps or in a continuous fashion,
respectively) following the input signal. Wasted heat on the output devices can be
reduced as excess voltage is kept to a minimum. The amplifier that is fed with
these rails itself can be of any class. These kinds of amplifiers are more complex,
and are mainly used for specialized applications, such as very high-power units.
Also, class E and class F amplifiers are commonly described in literature for radio
frequencies applications where efficiency of the traditional classes in are
important, yet several aspects not covered elsewhere (e.g.: amplifiers often simply
said to have a gain of x dB - so what power gain?) deviate substantially from their
ideal values. These classes use harmonic tuning of their output networks to
achieve higher efficiency and can be considered a subset of Class C due to their
conduction angle characteristics.
The classes can be most easily understood using the diagrams in each section below. For
the sake of illustration, a bipolar junction transistor is shown as the amplifying device,
but in practice this could be a MOSFET or vacuum tube device. In an analog amplifier
(the most common kind), the signal is applied to the input terminal of the device (base,
gate or grid), and this causes a proportional output drive current to flow out of the output
terminal. The output drive current comes from the power supply.
Class A amplifier
Amplifying devices operating in class A conduct over the whole of the input cycle such
that the output signal is an exact scaled-up replica of the input with no clipping. A class A
amplifier is distinguished by the output stage being biased into class A (see definition
Advantages of class A amplifiers
# Class A designs are simpler than other classes; for example class AB and B designs
require two devices (push-pull output) to handle both halves of the waveform; class A
can use a single device single-ended.
# The amplifying element is biased so the device is always conducting to some extent,
normally implying the quiescent (small-signal) collector current (for transistors; drain
current for FETs or anode/plate current for vacuum tubes) is close to the most linear
portion of its transconductance curve.
# Because the device is never shut off completely there is no "turn on" time, little
problem with charge storage, and generally better high frequency performance and
feedback loop stability (and usually fewer high-order harmonics).
# The point at which the device comes closest to being cut off is not close to zero signal,
so the problem of crossover distortion associated with class AB and B designs is avoided.
Disadvantage of class A amplifiers
# They are very inefficient; a theoretical maximum of 50% is obtainable with inductive
output coupling and only 25% with capacitive coupling, unless deliberate use of
nonlinearities is made (such as in square-law output stages). In a power amplifier this not
only wastes power and limits battery operation, it may place restrictions on the output
devices that can be used (for example: ruling out some audio triodes if modern low-
efficiency loudspeakers are to be used), and will increase costs. Inefficiency comes not
just from the fact that the device is always conducting to some extent (that happens even
with class AB, yet its efficiency can be close to that of class B); it is that the standing
current is roughly half the maximum output current (although this can be less with square
law output stage), together with the problem that a large part of the power supply voltage
is developed across the output device at low signal levels (as with classes AB and B, but
unlike output stages such as class D). If high output powers are needed from a class A
circuit, the power waste (and the accompanying heat) will become significant. For every
watt delivered to the load, the amplifier itself will, at best, dissipate another watt. For
large powers this means very large and expensive power supplies and heat sinking.
Class A designs have largely been superseded by the more efficient designs for power
amplifiers, though they remain popular with some hobbyists, mostly for their simplicity.
Also, many audiophiles believe that class A gives the best sound quality (for their
absence of crossover distortion and reduced odd-harmonic and high-order harmonic
distortion) which provides a small market for expensive high fidelity class A amps.
Single-ended and triode class A amplifiers
Some aficionados who prefer class A amplifiers also prefer the use of thermionic valve
(or "tube") designs instead of transistors, especially in Single-ended triode output
configurations for several claimed reasons:
# Single-ended output stages (be they tube or transistor) have an asymmetrical transfer
function, meaning that even order harmonics in the created distortion tend not to be
canceled (as they are in push-pull output stages); by using tubes OR FETs most of the
distortion is from the square law transfer characteristic and so second-order, which some
consider to be "warmer" and more pleasant.
# For those who prefer low distortion figures, the use of tubes with class A (generating
little odd-harmonic distortion, as mentioned above) together with symmetrical circuits
(such as push-pull output stages, or balanced low-level stages) results in the cancellation
of most of the even distortion harmonics, hence the removal of most of the distortion.
# Though good amplifier design can reduce harmonic distortion patterns to almost
nothing, distortion is essential to the sound of electric guitar amplifiers, for example, and
is held by recording engineers to offer more flattering microphones and to enhance
"clinical-sounding" digital technology.
# Historically, valve amplifiers often used a class A power amplifier simply because
valves are large and expensive; many class A designs use only a single device.
Transistors are much cheaper, and so more elaborate designs that give greater efficiency
but use more parts are still cost-effective. A classic application for a pair of class A
devices is the long-tailed pair, which is exceptionally linear, and forms the basis of many
more complex circuits, including many audio amplifiers and almost all op-amps. Class A
amplifiers are often used in output stages of high quality op-amps (although the accuracy
of the bias in low cost op-amps such as the 741 may result in class A or class AB or class
B, varying from device to device or with temperature). They are sometimes used as
medium-power, low-efficiency, and high-cost audio amplifiers. The power consumption
is unrelated to the output power. At idle (no input), the power consumption is essentially
the same as at high output volume. The result is low efficiency and high heat dissipation.
Class B and AB
Class B or AB push–pull circuits are the most common design type found in audio power
amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since
much of the time the music is quiet enough that the signal stays in the "class A" region,
where it is amplified with good fidelity, and by definition if passing out of this region, is
large enough that the distortion products typical of class B are relatively small. The
crossover distortion can be reduced further by using negative feedback. Class B and AB
amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are
also favored in battery-operated devices, such as transistor radios.
Class B amplifier
Class B amplifiers only amplify half of the input wave cycle, thus creating a large amount
of distortion, but their efficiency is greatly improved and is much better than class A.
Class B has a maximum theoretical efficiency of π/4. (i.e. 78.5%) This is because the
amplifying element is switched off altogether half of the time, and so cannot dissipate
power. A single class B element is rarely found in practice, though it has been used for
driving the loudspeaker in the early IBM Personal Computers with beeps, and it can be
used in RF power amplifier where the distortion levels are less important. However, class
C is more commonly used for this.
A practical circuit using class B elements is the push-pull stage, such as the very
simplified complementary pair arrangement shown below. Here, complementary or
quasi-complementary devices are each used for amplifying the opposite halves of the
input signal, which is then recombined at the output. This arrangement gives excellent
efficiency, but can suffer from the drawback that there is a small mismatch in the cross-
over region - at the "joins" between the two halves of the signal, as one output device has
to take over supplying power exactly as the other finishes. This is called crossover
distortion. An improvement is to bias the devices so they are not completely off when
they're not in use. This approach is called class AB operation.
Class B push–pull amplifier
In class AB operation, each device operates the same way as in class B over half the
waveform, but also conducts a small amount on the other half. As a result, the region
where both devices simultaneously are nearly off (the "dead zone") is reduced. The result
is that when the waveforms from the two devices are combined, the crossover is greatly
minimised or eliminated altogether. The exact choice of quiescent current, the standing
current through both devices when there is no signal, makes a large difference to the level
of distortion (and to the risk of thermal runaway, that may damage the devices); often the
bias voltage applied to set this quiescent current has to be adjusted with the temperature
of the output transistors (for example in the circuit at the beginning of the article the
diodes would be mounted physically close to the output transistors, and chosen to have a
matched temperature coefficient). Another approach (often used as well as thermally
tracking bias voltages) is to include small value resistors in series with the emitters.
Class AB sacrifices some efficiency over class B in favor of linearity, thus is less
efficient (below 78.5% for full-amplitude sinewaves in transistor amplifiers, typically;
much less is common in class AB vacuum tube amplifiers). It is typically much more
efficient than class A.
Class C amplifier
Class C amplifiers conduct less than 50% of the input signal and the distortion at the
output is high, but high efficiencies (up to 90%) are possible. Some applications (for
example, megaphones) can tolerate the distortion. A much more common application for
class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by
using tuned loads on the amplifier stage. The input signal is used to roughly switch the
amplifying device on and off, which causes pulses of current to flow through a tuned
The class C amplifier has two modes of operation: tuned and untuned. The diagram
shows a waveform from a simple class C circuit without the tuned load. This is called
untuned operation, and the analysis of the waveforms shows the massive distortion that
appears in the signal. When the proper load (e.g., a pure inductive-capacitive filter) is
used, two things happen. The first is that the output's bias level is clamped, so that the
output variation is centered at one-half of the supply voltage. This is why tuned operation
is sometimes called a clamper. This action of elevating bias level allows the waveform to
be restored to its proper shape, allowing a complete waveform to be re-established
despite having only a one-polarity supply. This is directly related to the second
phenomenon: the waveform on the center frequency becomes much less distorted. The
distortion that is present is dependent upon the bandwidth of the tuned load, with the
center frequency seeing very little distortion, but greater attenuation the farther from the
tuned frequency that the signal gets.
The tuned circuit will only resonate at one frequency, and so the unwanted frequencies
are suppressed, and the wanted full signal (sine wave) will be extracted by the tuned load.
The signal bandwith of the amplifier is limited by the Q-factor of the tuned circuit but
this is not a serious limitation. Any residual harmonics can be removed using a further
In practical class-C amplifiers a tuned load is invariably used. In one common
arrangement the resistor shown in the circuit above is replaced with a parallel-tuned
circuit consisting of an inductor and capacitor in parallel, whose components are chosen
to resonate the frequency of the input signal. Power can be coupled to a load by
transformer action with a secondary coil wound on the inductor. The average voltage at
the drain is then equal to the supply voltage, and the signal voltage appearing across the
tuned circuit varies from near zero to near twice the supply voltage during the rf cycle.
The input circuit is biassed so that the active element (e.g. transistor) conducts for only a
fraction of the rf cycle, usually one third (120 degrees) or less.
The active element conducts only while the drain voltage is passing through its minimum.
By this means, power dissipation in the active device is minimised, and efficiency
increased. Ideally, the active element would pass only an instantaneous current pulse
while the voltage across it is zero: it then disspates no power and 100% efficiency is
achieved. However practical devices have a limit to the peak current they can pass, and
the pulse must therefore be widened, to around 120 degrees, to obtain a reasonable
amount of power, and the efficiency is then 60-7[
Main article: Class D amplifier
In the class D amplifier the input signal is converted to a sequence of higher voltage
output pulses. The averaged-over-time power values of these pulses are directly
proportional to the instantaneous amplitude of the input signal. The frequency of the
output pulses is typically ten or more times the highest frequency in the input signal to be
amplified. The output pulses contain inaccurate spectral components (that is, the pulse
frequency and its harmonics) which must be removed by a low-pass passive filter. The
resulting filtered signal is then an amplified replica of the input.
These amplifiers use pulse width modulation, pulse density modulation (sometimes
referred to as pulse frequency modulation) or more advanced form of modulation such as
Delta-sigma modulation (for example, in the Analog Devices AD1990 class D audio
power amplifier). Output stages such as those used in pulse generators are examples of
class D amplifiers. The term class D is usually applied to devices intended to reproduce
signals with a bandwidth well below the switching frequency.
Class D amplifiers can be controlled by either analog or digital circuits. The digital
control introduces additional distortion called quantization error caused by its conversion
of the input signal to a digital value.
The main advantage of a class D amplifier is power efficiency. Because the output pulses
have a fixed amplitude, the switching elements (usually MOSFETs, but valves (a.k.a
vacuum tubes) and bipolar transistors were once used) are switched either completely on
or completely off, rather than operated in linear mode. A MOSFET operates with the
lowest resistance when fully on and thus has the lowest power dissipation when in that
condition, except when fully off. When operated in a linear mode the MOSFET has
variable amounts of resistance that vary linearly with the input voltage and the resistance
is something other than the minimum possible, therefore more electrical energy is
dissipated as heat. Compared to class A/B operation, class D's lower losses permit the use
of a smaller heat sink for the MOSFETS while also reducing the amount of AC power
supply power required. Thus, class D amplifiers do not need as large or as heavy power
supply transformers or heatsinks, so they are smaller and more compact in size than an
equivalent class AB amplifier.
Class D amplifiers have been widely used to control motors, and almost exclusively for
small DC motors, but they are now also used as audio amplifiers, with some extra
circuitry to allow analogue to be converted to a much higher frequency pulse width
modulated signal. The relative difficulty of achieving good audio quality means that
nearly all are used in applications where quality is not a factor, such as modestly priced
bookshelf audio systems and "DVD-receivers" in mid-price home theater systems.
High quality class D audio amplifiers have now appeared in the market and these revised
designs have been said to rival good traditional AB amplifiers in terms of quality. Before
these higher quality designs existed an earlier use of class D amplifiers and prolific area
of application was high-powered, subwoofer amplifiers in cars. Because subwoofers are
generally limited to a bandwidth of no higher than 150 Hz, the switching speed for the
amplifier does not have to be as high as for a full range amplifier. Class D amplifiers for
driving subwoofers are relatively inexpensive, in comparison to class AB amplifiers.
The letter D used to designate this amplifier class is simply the next letter after C, and
does not stand for digital. Class D and class E amplifiers are sometimes mistakenly
described as "digital" because the output waveform superficially resembles a pulse-train
of digital symbols, but a class D amplifier merely converts an input waveform into a
continuously pulse-width modulated (square wave) analog signal. (A digital waveform
would be pulse-code modulated.)
The class E/F amplifier is a highly efficient switching power amplifier, typically used at
such high frequencies that the switching time becomes comparable to the duty time. As
said in the class D amplifier, the transistor is connected via a serial LC circuit to the load,
and connected via a large L (inductor) to the supply voltage. The supply voltage is
connected to ground via a large capacitor to prevent any RF signals leaking into the
supply. The class E amplifier adds a C (capacitor) between the transistor and ground and
uses a defined L1 to connect to the supply voltage.
Class E amplifier
The following description ignores DC, which can be added easily afterwards. The above
mentioned C and L are in effect a parallel LC circuit to ground. When the transistor is on,
it pushes through the serial LC circuit into the load and some current begins to flow to the
parallel LC circuit to ground. Then the serial LC circuit swings back and compensates the
current into the parallel LC circuit. At this point the current through the transistor is zero
and it is switched off. Both LC circuits are now filled with energy in C and L0. The whole
circuit performs a damped oscillation. The damping by the load has been adjusted so that
some time later the energy from the Ls is gone into the load, but the energy in both C0
peaks at the original value to in turn restore the original voltage so that the voltage across
the transistor is zero again and it can be switched on.
With load, frequency, and duty cycle (0.5) as given parameters and the constraint that the
voltage is not only restored, but peaks at the original voltage, the four parameters (L, L0,
C and C0) are determined. The class E amplifier takes the finite on resistance into account
and tries to make the current touch the bottom at zero. This means that the voltage and
the current at the transistor are symmetric with respect to time. The Fourier transform
allows an elegant formulation to generate the complicated LC networks and says that the
first harmonic is passed into the load, all even harmonics are shorted and all higher odd
harmonics are open.
Class E uses a significant amount of second-harmonic voltage. The second harmonic can
be used to reduce the overlap with edges with finite sharpness. For this to work, energy
on the second harmonic has to flow from the load into the transistor, and no source for
this is visible in the circuit diagram. In reality, the impedance is mostly reactive and the
only reason for it is that class E is a class F (see below) amplifier with a much simplified
load network and thus has to deal with imperfections.
In many amateur simulations of class E amplifiers, sharp current edges are assumed
nullifying the very motivation for class E and measurements near the transit frequency of
the transistors show very symmetric curves, which look much similar to class F
The class E amplifier was invented in 1972 by Nathan O. Sokal and Alan D. Sokal, and
details were first published in 1975. Some earlier reports on this operating class have
been published in Russian.
In push-pull amplifiers and in CMOS, the even harmonics of both transistors just cancel.
Experiment shows that a square wave can be generated by those amplifiers. Theoretically
square waves consist of odd harmonics only. In a class D amplifier, the output filter
blocks all harmonics; i.e., the harmonics see an open load. So even small currents in the
harmonics suffice to generate a voltage square wave. The current is in phase with the
voltage applied to the filter, but the voltage across the transistors is out of phase.
Therefore, there is a minimal overlap between current through the transistors and voltage
across the transistors. The sharper the edges, the lower the overlap.
while in class D, transistors and the load exist as two separate modules, class F admits
imperfections like the parasitics of the transistor and tries to optimise the global system to
have a high impedance at the harmonics. Of course there has to be a finite voltage across
the transistor to push the current across the on-state resistance. Because the combined
current through both transistors is mostly in the first harmonic, it looks like a sine. That
means that in the middle of the square the maximum of current has to flow, so it may
make sense to have a dip in the square or in other words to allow some overswing of the
voltage square wave. A class F load network by definition has to transmit below a cutoff
frequency and reflect above.
Any frequency lying below the cutoff and having its second harmonic above the cutoff
can be amplified, that is an octave bandwidth. On the other hand, an inductive-capacitive
series circuit with a large inductance and a tunable capacitance may be simpler to
implement. By reducing the duty cycle below 0.5, the output amplitude can be
modulated. The voltage square waveform will degrade, but any overheating is
compensated by the lower overall power flowing. Any load mismatch behind the filter
can only act on the first harmonic current waveform, clearly only a purely resistive load
makes sense, then the lower the resistance, the higher the current.
Class F can be driven by sine or by a square wave, for a sine the input can be tuned by an
inductor to increase gain. If class F is implemented with a single transistor, the filter is
complicated to short the even harmonics. All previous designs use sharp edges to
minimise the overlap.
Classes G and H
There are a variety of amplifier designs that enhance class AB output stages with more
efficient techniques to achieve greater efficiencies with low distortion. These designs are
common in large audio amplifiers since the heatsinks and power transformers would be
prohibitively large (and costly) without the efficiency increases. The terms "class G" and
"class H" are used interchangeably to refer to different designs, varying in definition from
one manufacturer or paper to another.
Class G amplifiers (which use "rail switching" to decrease power consumption and
increase efficiency) are more efficient than class AB amplifiers. These amplifiers provide
several power rails at different voltages and switch between them as the signal output
approaches each level. Thus, the amplifier increases efficiency by reducing the wasted
power at the output transistors. Class G amplifiers are more efficient than class AB but
less efficient when compared to class D, without the negative EMI effects of class D.
Class H amplifiers take the idea of class G one step further creating an infinitely variable
supply rail. This is done by modulating the supply rails so that the rails are only a few
volts larger than the output signal at any given time. The output stage operates at its
maximum efficiency all the time. Switched-mode power supplies can be used to create
the tracking rails. Significant efficiency gains can be achieved but with the drawback of
more complicated supply design and reduced THD performance.
The voltage signal shown is thus a larger version of the input, but has been changed in
sign (inverted) by the amplification. Other arrangements of amplifying device are
possible, but that given (that is, common emitter, common source or common cathode) is
the easiest to understand and employ in practice. If the amplifying element is linear, then
the output will be faithful copy of the input, only larger and inverted. In practice,
transistors are not linear, and the output will only approximate the input. Non-linearity
from any of several sources is the origin of distortion within an amplifier. Which class of
amplifier (A, B, AB or C) depends on how the amplifying device is biased — in the
diagrams the bias circuits are omitted for clarity.
Any real amplifier is an imperfect realization of an ideal amplifier. One important
limitation of a real amplifier is that the output it can generate is ultimately limited by the
power available from the power supply. An amplifier will saturate and clip the output if
the input signal becomes too large for the amplifier to reproduce or if operational limits
for a device are exceeded.
For additional information on class H: Efficiency Class H
A hybrid configuration receiving new attention is the Doherty amplifier, invented in 1934
by William H. Doherty for Bell Laboratories (whose sister company, Western Electric,
was then an important manufacturer of radio transmitters). The Doherty amplifier
consists of a class B primary or carrier stage in parallel with a class C auxiliary or peak
stage. The input signal is split to drive the two amplifiers and a combining network sums
the two output signals. Phase shifting networks are employed in the inputs and the
outputs. During periods of low signal level, the class B amplifier efficiently operates on
the signal and the class C amplifier is cutoff and consumes little power. During periods of
high signal level, the class B amplifier delivers its maximum power and the class C
amplifier delivers up to its maximum power. The efficiency of previous AM transmitter
designs was proportional to modulation but, with average modulation typically around
20%, transmitters were limited to less than 50% efficiency. In Doherty's design, even
with zero modulation, a transmitter could achieve at least 60% efficiency.
As a successor to Western Electric for broadcast transmitters, the Doherty concept was
considerably refined by Continental Electronics Manufacturing Company of Dallas, TX.
Perhaps, the ultimate refinement was the screen-grid modulation scheme invented by
Joseph B. Sainton. The Sainton amplifier consists of a class C primary or carrier stage in
parallel with a class C auxiliary or peak stage. The stages are split and combined through
90-degree phase shifting networks as in the Doherty amplifier. The unmodulated radio
frequency carrier is applied to the control grids of both tubes. Carrier modulation is
applied to the screen grids of both tubes. The bias point of the carrier and peak tubes is
different, and is established such that the peak tube is cutoff when modulation is absent
(and the amplifier is producing rated unmodulated carrier power) whereas both tubes
contribute twice the rated carrier power during 100% modulation (as four times the
carrier power is required to achieve 100% modulation). As both tubes operate in class C,
a significant improvement in efficiency is thereby achieved in the final stage. In addition,
as the tetrode carrier and peak tubes require very little drive power, a significant
improvement in efficiency within the driver stage is achieved as well (317C, et al.).
The released version of the Sainton amplifier employs a cathode-follower modulator, not
a push-pull modulator. Previous Continental Electronics designs, by James O. Weldon
and others, retained most of the characteristics of the Doherty amplifier but added screen-
grid modulation of the driver (317B, et al.).
The Doherty amplifier remains in use in very-high-power AM transmitters, but for lower-
power AM transmitters, vacuum-tube amplifiers in general were eclipsed in the 1980s by
arrays of solid-state amplifiers, which could be switched on and off with much finer
granularity in response to the requirements of the input audio. However, interest in the
Doherty configuration has been revived by cellular-telephone and wireless-Internet
applications where the sum of several constant-envelope users creates an aggregate AM
result. The main challenge of the Doherty amplifier for digital transmission modes is in
aligning the two stages and getting the class-C amplifier to turn on and off very quickly.
Recently, Doherty amplifiers have found widespread use in cellular base station
transmitters for GHz frequencies. Implementations for transmitters in mobile devices
have also been demonstrated.
Various newer classes of amplifier, as defined by the technical details of their topology,
have been developed on the basis of previously existing operating classes. For example,
Crown's K and I-Tech Series as well as several other models utilise Crown's patented
class I (or BCA) technology. Lab.gruppen use a form of class D amplifier called class TD
or tracked class D which tracks the waveform to more accurately amplify it without the
drawbacks of traditional class D amplifiers.
"Class T" was a trademark of TriPath company which manufactures audio amplifier ICs.
This new class T is a revision of the common class D amplifier, but with changes to
ensure fidelity over the full audio spectrum, unlike traditional class D designs. It operates
at different frequencies depending on the power output, with values ranging from as low
as 200 kHz to 1.2 MHz, using a proprietary modulator. Tripath ceased
operations in 2007, its patents acquired by Cirrus Logic for their Mixed-Signal Audio
division. Some Kenwood Recorder use Class W amplifier 
"Class Z" is a trademark of Zetex Semiconductors (now part of Diodes Inc. of Dallas,
TX) and is a direct-digital-feedback technology. Zetex-patented circuits are being
utilised in the latest power amplifiers by NAD Electronics of Canada.
The practical amplifier circuit to the right could be the basis for a moderate-power audio
amplifier. It features a typical (though substantially simplified) design as found in
modern amplifiers, with a class AB push–pull output stage, and uses some overall
negative feedback. Bipolar transistors are shown, but this design would also be realizable
with FETs or valves.
The input signal is coupled through capacitor C1 to the base of transistor Q1. The
capacitor allows the AC signal to pass, but blocks the DC bias voltage established by
resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form a
differential amplifier (an amplifier that multiplies the difference between two inputs by
some constant), in an arrangement known as a long-tailed pair. This arrangement is used
to conveniently allow the use of negative feedback, which is fed from the output to Q2
via R7 and R8.
The negative feedback into the difference amplifier allows the amplifier to compare the
input to the actual output. The amplified signal from Q1 is directly fed to the second
stage, Q3, which is a common emitter stage that provides further amplification of the
signal and the DC bias for the output stages, Q4 and Q5. R6 provides the load for Q3 (A
better design would probably use some form of active load here, such as a constant-
current sink). So far, all of the amplifier is operating in class A. The output pair are
arranged in class AB push–pull, also called a complementary pair. They provide the
majority of the current amplification (while consuming low quiescent current) and
directly drive the load, connected via DC-blocking capacitor C2. The diodes D1 and D2
provide a small amount of constant voltage bias for the output pair, just biasing them into
the conducting state so that crossover distortion is minimized. That is, the diodes push the
output stage firmly into class-AB mode (assuming that the base-emitter drop of the
output transistors is reduced by heat dissipation).
This design is simple, but a good basis for a practical design because it automatically
stabilises its operating point, since feedback internally operates from DC up through the
audio range and beyond. Further circuit elements would probably be found in a real
design that would roll off the frequency response above the needed range to prevent the
possibility of unwanted oscillation. Also, the use of fixed diode bias as shown here can
cause problems if the diodes are not both electrically and thermally matched to the output
transistors — if the output transistors turn on too much, they can easily overheat and
destroy themselves, as the full current from the power supply is not limited at this stage.
A common solution to help stabilise the output devices is to include some emitter
resistors, typically an ohm or so. Calculating the values of the circuit's resistors and
capacitors is done based on the components employed and the intended use of the amp.
For the basics of radio frequency amplifiers using valves, see Valved RF amplifiers.
Notes on implementation
Real world amplifiers are imperfect.
# One consequence is that the power supply itself may influence the output, and must
itself be considered when designing the amplifier
# The amplifier circuit has an "open loop" performance, that can be described as various
parameters (gain, slew rate, output impedance, distortion, bandwidth, signal to noise
# Many modern amplifiers use negative feedback techniques to hold the gain at the
Different methods of supplying power result in many different methods of bias. Bias is a
technique by which the active devices are set up to operate in a particular regime, or by
which the DC component of the output signal is set to the midpoint between the
maximum voltages available from the power supply. Most amplifiers use several devices
at each stage; they are typically matched in specifications except for polarity. Matched
inverted polarity devices are called complementary pairs. Class A amplifiers generally
use only one device, unless the power supply is set to provide both positive and negative
voltages, in which case a dual device symmetrical design may be used. Class C
amplifiers, by definition, use a single polarity supply.
Amplifiers often have multiple stages in cascade to increase gain. Each stage of these
designs may be a different type of amp to suit the needs of that stage. For instance, the
first stage might be a class A stage, feeding a class AB push–pull second stage, which
then drives a class G final output stage, taking advantage of the strengths of each type,
while minimizing their weaknesses.S